The British Prime Minister has indicated the UK Government is paving the way for airstrikes on Islamic State terrorists in Syria in the wake of the Tunisian beach massacre, in which 30 Britons and 3 Irish people were murdered.
David Cameron has said there is a "strong case for us doing more in Syria" but said there needed to be "clear consensus for such action" among MPs.
Defence Secretary Michael Fallon is expected to make the case for allowing RAF bombers to attack key IS targets ahead of a debate on the issue in the House of Commons.
Mr Cameron has promised no military action without putting it to the Commons, however, while MPs are not expected to be asked to vote on the issue today, his comment suggest a vote will be imminent.
MPs voted for airstrikes in Iraq last year but they were not asked to approve wider action to include Syria.
But Mr Fallon has said it is "illogical" that UK forces were able to strike against the terrorists in Iraq but not in Syria.
Mr Cameron suffered a damaging defeat over military action against Bashar Assad in Syria in 2013 when Ed Miliband led a vote against intervention that saw 30 Tory MPs rebel.
However, the situation in the country has changed considerably and the Government has indicated it is preparing for policy change on the issue.
It comes after Mr Cameron promised a "full spectrum" response to the murder of 30 Britons by an IS gunman last week.
The debate in the Commons later today will give ministers a chance to gauge the position of MPs and assess whether a vote on air strikes will be successful.
The Prime Minister's official spokesman said: "The Prime Minister has long thought that ISIL poses a threat to Britain and ISIL needs to be destroyed in Syria and Iraq.
"He has said there is a strong case for us doing more in Syria but he's also been clear he thinks it would be better if there was a consensus for such action in the House.
"What changes is the growing evidence that ISIL poses a threat to our national security. In that context MPs should be thinking about these issues."
He added: "When you're faced with 30 British people murdered by a gunman where they are investigating links to ISIL, does that underline the real threat posed to us here in Britain? I think it does."
Mr Fallon said: "It is a new parliament and MPs will want to think very carefully about how we best deal with Isil. [There is an] illogicality [with] Isil not respecting the border lines; they don’t differentiate between Syria and Iraq, they're establishing this evil caliphate across both countries. There is no legal bar to us operating in Syria but we don’t have the parliamentary approval for it.
"We don't need it at the moment because we are playing our part in the campaign and what we do in Iraq actually frees up the US aircraft to attack in Syria. Isil has to be defeated in both countries. Its evil in Iraq is all being directed by its headquarters in Syria."
Speaking to Sky News Col Bob Stewart, the former UN Commander in Bosnia, said he would be in favour of the UK taking action in Syria because that is where the key strategic targets are.
He said the "world has allowed these people to terrorise huge groups of innocents".
He said: "I would very much prefer that the Arab world would get on with this and sort it out because the only way to beat the Daesh (IS) thugs is actually to beat them on the ground.
"I don't see why we should once again have to do it. We are already contributing by using air power and intelligence assets but I think the Arab world should do it.
"That's the plan but it's not working."
However, Conservative MP Crispin Blunt, who is chairman of the influential House of Commons Foreign Affairs Committee, said there was no military necessity for strikes in Syria.
He told the Radio 4 Today programme: "I would want to know whether this is a battle-winning decision. Plainly, the United Kingdom role in all this is pretty minor, and we should be concentrating on getting the battle-winning decision - which is actually getting the regional states to co-operate around the mission, which is to defeat Isil.
"There's no military necessity for this. We are not providing very many of the aircraft. Five per cent of the missions are being flown by the United Kingdom.
"Therefore it makes no practical difference, and we are getting ourselves in to a slightly more legal grey area. I don't think it's as clear as people have said. It's easy to come in as guests of the Government of Iraq at their invitation in their country. It becomes slightly more questionable when you don't have a UN Security Council resolution and you are operating in another country."
